It is well known in the art of hydroforming hollow metal parts to use the hydroforming pressure to assist a punch in piercing one or more holes in the hydroformed part while the part remains in the hydroforming die. The hydroformed part can take various shapes as for example a chassis frame and engine cradle for a motor vehicle wherein one or more holes are required in these hydroformed parts for the attachment of one or more other frame or cradle forming parts respectively. This piercing operation with a punch can produce a detached slug left in the hydroformed part that may or may not be undesirable. In the latter case and depending on whether a single hole is punched or aligned holes in opposite sides of a hydroformed part are punched, the detached slugs in either situation can be removed as disclosed in U.S. Pat. No. 5,398,533 issued Mar. 21, 1995 and U.S. Pat. No. 5,666,840 issued Sep. 16, 1997 which are both assigned to the assignee of this invention.
Is also well known in the case of where a loose slug left in the hydroformed part is undesirable, that the piercing can be accomplished with a punch that produces a slug that remains integral with the hydroformed part. This is shown and labeled as "Prior Art" in FIG. 1 of the accompanying drawings. In this case, as in other hydroforming operations, a hollow metal part 10 has initially been forced by hydraulic fluid 11 at high pressure to conform to the surface of a die cavity 12 in a hydroforming die 14 comprising mating die halves 16 and 18. A punch 20 mounted for reciprocal movement in a cylindrical bore 22 in the upper die half 16 has, as distinguished from the above cited U.S. Patents, a chamfered or beveled portion 24 of limited peripheral extent that interrupts the sharp peripherally extending cutting edge 26 at the end face 28 of the punch. The chamfered portion 24 extends through a small arc about the periphery of the punch face, for example 45 degrees, and as a result, when the punch is advanced into the part while the internal hydraulic pressure is maintained and cutting of the part takes place with the remaining cutting edge 26, a small portion 30 of the slug 32 that is produced remains intact or integral with the hydroformed part and is forced with the slug by the punch face into the interior of the hydroformed part.
In the above cited U.S. Patents and FIG. 1, the punches illustrated produce circular holes in the hydroformed part. However, it is also well known by those skilled in this art that these punches as well as punches in general can be made to produce various hole shapes such as square, rectangular, triangular or other common geometrical shapes or an irregular shape as required for a particular hydroformed part. In either event and as particularly concerns the piercing of hydroformed parts, there are limitations on the size of the hole that can be punched and particularly where the slug is left attached to the side of the hole as in the case of the Prior Art in FIG. 1. For example and again referring to FIG. 1, where any internal cross sectional dimension A of the hydroformed part 10 is less than dimension B of the slug 32 that lies in the same plane with the former dimension and in this case is the diameter of the punch, the slug can hit the opposite interior side of the part as the slug is punched as shown in FIG. 1. This can produce several undesirable results such as is the case with the slug 32 obstructing the hole as shown and thereby shortening the depth of the hole which could be unacceptable for the reception of a mating part (not shown). The worst condition has been found to be a miss-shaped hole with a very distorted side and results from the slug hitting the interior of the part and then being further bent by the punch as shown by the deformed portion 30A of the slug illustrated with phantom lines.